In a trifle dish or large glass salad bowl layer, black beans, green chiles, corn, lettuce, tomatoes, cheese, olives, and scallions. Save a few tomatoes and scallions to decorate the top. In another bowl mix together the guacamole, sour cream and garlic powder. Carefully spread over the top of the salad. Cover the bowl with plastic wrap and refrigerate until ready to serve. You can also serve with a couple cups of crushed tortilla chips on top or serve the chips on the side.